The strategic review began Oct. 12 after billionaire investor Carl Icahn bought up shares of Biogen Idec, which analysts expected could have fetched a price of more than $20 billion. But the Cambridge-based company said Wednesday that a search for a possible buyer failed to yield any definitive offers.

Icahn's investment led to speculation that he could be preparing a buyout bid or might seek to increase the value of his shares by encouraging a higher bid by a major pharmaceutical company.

But Biogen Idec said after markets closed Wednesday that its board had decided the firm "will continue on its present course as an independent company."

The maker of drugs to treat conditions including multiple sclerosis and arthritis said "it is confident that continued execution of the company's business plan will result in attractive value for stockholders."

After closing up 49 cents at $75.88 in the regular session, Biogen Idec shares fell $20.37, or 26.9 percent, to $55.51 in after-hours trading -- nearly $14 below the closing price two months ago, just before Biogen Idec announced it would review a possible sale, when the company's market value stood at about $20 billion. Wednesday's after-hours decline left the stock at about the same level it had been before Icahn's investment triggered a late summer run-up in the stock's price.

Cowen & Co. analyst Eric Schmidt said the stock's recent rise could have scared away potential buyers who found the company too pricey.

Schmidt said potential buyers also may have held off because of Biogen Idec's partnerships with Genentech Inc. and Elan Corp. for some of its key drugs -- relationships that could have created complications for any buyer.

As for Icahn's failure to submit a definitive offer, Schmidt said, "I don't think Icahn was ever serious about buying at any price. I don't think he's in the business of buying biotech companies for around $20 billion."

Deutsche Bank analyst Jennifer Chao said lingering safety concerns about Biogen Idec's MS drug Tysabri also could have scared off potential buyers.

Biogen Idec became the second Boston area biotech firm this month to commit to remaining independent after investments by Icahn. Last week, the chief executive of Genzyme Corp. said he had no interest in selling his company, despite speculation that Icahn's recent purchase of Genzyme shares could trigger a sale.

Icahn often pushes for major changes at companies he invests in, and his threat of a shareholder fight at MedImmune Inc. preceded the purchase of the Gaithersburg, Md.-based drug maker by AstraZeneca PLC for $15.6 billion in April.

In Biogen Idec's case, Wall Street analysts speculated that potential buyers could include Pfizer Inc., GlaxoSmithKline PLC, Sanofi-Aventis SA, and Novartis AG.

A couple weeks after the review began, Biogen Chief Executive James Mullen called the uncertainty about a sale "unsettling" for Biogen's nearly 4,000 employees and said he hoped to conclude the process "as quickly as is reasonable."

In August, federal antitrust regulators cleared Icahn to buy a stake. Earlier in the month, Icahn reported purchasing 2.7 million Biogen shares as of June 30.

The 29-year-old company's recent fortunes have been tied heavily to Tysabri, which was withdrawn from the market in February 2005 because of safety concerns.

Tysabri had been seen as a potential blockbuster for Biogen Idec and Irish partner Elan before it was withdrawn after two clinical trial patients died of a rare brain disorder. U.S. and European authorities approved Tysabri's reintroduction in June 2006, but with restrictions to monitor patients for symptoms of the brain disease. About 17,000 patients were on Tysabri worldwide as of the end of September, and the company expects more than 100,000 by the end of 2010.

Biogen Idec's other key drugs include Avonex and Rituxan, approved for non-Hodgkin's lymphoma and rheumatoid arthritis.